During 2012 RPVs inspections, nearly-laminar indications were detected in the lower and upper core shells of Doel 3 and Tihange 2 RPVs. As a consequence, the Doel 3 and Tihange 2 NPPs have to stay in cold shutdown until it was proved that they can be safely operated. The performance and capability of the applied UT technique to detect and characterize hydrogen flakes had to be confirmed. To achieve this objective, extensive investigations were launched on blocks extracted from a steam generator shell known for containing hydrogen flakes. The paper describes the UT validation performed and the results of the destructive tests which enable to demonstrate the capability of a straight ultrasonic beam to detect and size nearly laminar indications and to correctly identify ligaments between two adjacent indications. It addresses the formal extension of qualification performed by correlating UT measurements acquired on a large numbers of real hydrogen flakes, simulations of UT responses and destructive examinations. Lessons learned from the experiments are finally described.

No abstract